descriptive noun: The butthole who always insists on having the last word, regardless of the validity of his arguement. Will insist the earth is flat, even as he sails over the horizon.

He's such an Iknowbutt! I told him he'd electrocute himself with the hairdryer in the bathtub, but he just said, "I know, but if I am really careful, I can just paralyze myself and get a big settlement from the hairdryer company."